a. Field of the Invention
This invention relates to phosphorescent panels used to provide low level lighting for such activities as emergency evacuation, camping, hunting, military missions, novelty items, and other nighttime activities.
b. Related Art
The present invention addresses the need for brighter phosphorescent panels for such applications as camping, hunting, military missions, emergency building exit guides, night lighting in homes and business, and novelty uses. The invention produces substantially brighter emissions per unit area than even a completely solid layer of phosphorescent material, or than any previously described laminate structure, by enabling, capturing, and directing multiple emissions and reflections from phosphorescent particles (“phosphor”).
Phosphorescent materials absorb light that is incident upon them, raising the outermost electrons in their structure to higher energy levels. These electrons are left locked in these high energy levels, or orbitals, because it is a “spin forbidden” energy transition for them to drop back to their rest levels, that is, a change in electronic spin properties would be required, and this cannot readily occur. Because of this lock in to high-energy orbitals, the phosphor material remains “charged” for extended periods of time.
Over that extended period, second order transitions occur which allow the electrons to return to their resting energy orbitals, emitting light in the process. By this mechanism, phosphorescent materials absorb incident light and then emit over an extended period of time, in the range from minutes, to hours. The decay in light emission typically follows an exponential law, giving high emission initially, then rapidly dropping off over the first minute or so, then very slowly diminishing over a longer period of time.
The most common representative of traditional phosphorescent materials is zinc sulfide (ZnS), which provides a visible glow for a period of a few tens of minutes and is widely used in novelty items. The utility of this phosphorescent material has been limited because of its low light output and short duration of light output.
Improved phosphorescent materials that have recently been invented (e.g., U.S. Pat. No. 5,424,006) provide much greater light emission, and longer light emission, than do previous phosphor materials such as zinc sulfide. These improved phosphors, generally comprising strontium aluminate (SrAl2O4) or related compounds in crystals with various doping elements added, emit on the order of ten times as much light as ZnS, and do so for on the order of ten times as long. This increased light output creates opportunities for the development of many new applications for phosphorescent materials and is the critical advantage of these new phosphor materials.
A new class of phosphorescent material with greatly improved light output, duration of light output, and resistance to fading from ultraviolet light was disclosed by Murayama, et al in U.S. Pat. No. 5,424,006. These materials make possible a wide range of improved phosphorescent articles and extend the range of usefulness of said materials into new application fields. This patent does not, however, provide any suggestion for increasing the luminance by controlled distribution through a thick laminate structure.
Even with these increased light outputs, however, the light emitted by any phosphorescent material is still dim compared with typical room lighting levels, or even in comparison with moonlight. Any method of further increasing the brightness of light output and duration of light output from these materials is therefore highly desirable. A high brightness phosphorescent panel would increase the visibility of emergency building exit markers, aircraft low level lighting strips, household night light markers to avoid foot injuries and falls, camping and sporting lights, and military markers.
It is clear that the utility of phosphorescent products used as safety markers, emergency lighting, and for camping and military purposes is often tied to their intensity of light emission. This light emission, in turn, is a complex function of the phosphor particle size, the particle concentration, the particle distribution within the emitting body, and other geometric factors.
The prior art of fabrication of phosphorescent sheets and panels has been limited by a lack of understanding of these factors. For instance, U.S. Pat. No. 5,698,301 teaches the fabrication of a phosphorescent article with a thin phosphorescent layer with a very high concentration of phosphor powder. In particular, this patent cites a desirable thickness for a covering layer, a layer containing phosphorescent powder, and a reflective layer of between 70 and 600μ, with the thickness of the phosphor-bearing layer being preferably from 100 to 400μ. In fact, U.S. Pat. No. 5,698,301 specifically teaches that phosphor layer thickness above about 200μ are of no use because “no proportionate improvement in such characteristics as quickness of excitation, luminance, or afterglow property” will result from increasing thickness.
Furthermore, U.S. Pat. No. 5,698,301 cites a desirable concentration range for the phosphor powder in the carrier layer of between 70% to 85% by weight. It will be shown below that such a high concentration, while desirable in the thin layer taught in U.S. Pat. No. 5,698,301, would substantially reduce the brightness of the phosphor panel disclosed in the present invention.
Numerous other patents such as U.S. Pat. Nos. 5,674,554 and 6,048,595 teach the construction of similarly thin layers, on the order of 30 to 80μ, of high concentration phosphorescent powder. It will be shown that the present invention provides a significantly brighter phosphorescent laminate than any of these prior inventions, on the order of 200–300%, by exploiting an improved understanding of the relationship between surface brightness and phosphorescent particle size, concentration, distribution, panel thickness, and reflective backings. This improvement is of a somewhat surprising nature, contradicting the design principles outlined in the prior art, and was clearly not anticipated in any of the above, or other references.
Another typical thin phosphorescent layered structure is disclosed in U.S. Pat. No. 6,048,595, which describes a printed article with a printed phosphorescent substrate. The thickness of the substrate is 50 to 150 microns and the phosphor concentration is on the order of 50–80 weight percent (wt %). Such a thin phosphorescent layer will produce only a small fraction, on the order of 25–35% of the luminosity of the present invention.
Similarly, the phosphorescent laminate disclosed in U.S. Pat. No. 5,698,301 describes a phosphorescent layer that is about 50 to 200 microns thick and consists of 70 to 85 wt % phosphorescent material. Such a thickness and concentration of phosphorescent material produces only a single layer of phosphorescent emission, and results in an article only 25 to 40% as bright as the present invention. Nowhere in U.S. Pat. No. 5,698,301 is there any anticipation that deliberately separating phosphor particles by lowering the concentration, and building up many effective layers of low dilution phosphor particles, can result in a higher performance phosphorescent panel as disclosed in the present invention.
Similarly, a phosphorescent laminate is taught in U.S. Pat. No. 5,830,548, which describes methods for producing a calendared laminate structure panel, but said panels are coated with a phosphorescent material, rather than having such material distrubuted through the panel thickness for higher luminosity, as with the present invention.
Applications for luminescent panels are described by many patents, and attest to the usefulness of the present invention since most will benefit from the higher luminosity afforded by said invention. For example, U.S. Pat. No. 5,692,327 describes an application involving luminous license plates for vehicles.
A glow-in-the-dark commode is described in U.S. Pat. No. 6,279,180, which uses phosphorescent panels to provide better night time visualization. The patent does not describe the construction of said panels aside from mentioning an adhesive layer for affixing the panels.
A phosphorescent imaging panel is taught in U.S. Pat. No. 6,338,892. The invention involves an image receiving layer superimposed over a phosphorescent layer. The phosphorescent layer taught comprises a 1 mil thick layer of Luminova™ phosphorescent ink, said layer being substantially less bright than the present invention.
Phosphorescent novelty cards are disclosed in U.S. Pat. No. 5,997,992, having a printed layer over a phosphorescent layer. No mention is made of the details of the structure of the phosphorescent layer with regards to thickness, phosphor distribution, reflective backing layer, or resultant luminosity.
Phosphorescent signage is described in U.S. Pat. No. 6,358,563, and is produced by a phosphorescent paint with a castor oil carrier. Such paint would have insufficient thickness to generate the high luminosity of the present invention.
A fiber-based composite material is coated with a phosphorescent gel coat in U.S. Pat. No. 5,223,330. Again, the phosphorescent layer is very thin, being applied by silk screening or like means, and is insufficient to produce high luminosity by the mechanism of the present invention.
Similarly, an improved formulation for a composite gel coat material bearing a phosphorescent powder is disclosed in U.S. Pat. No. 6,207,077, but said gel coat layers are very thin and do not have the appropriate phosphor concentration to produce a high brightness laminate. Nowhere in that disclosure is there any anticipation that emission brightness can be enhanced by the mechanism of the present invention.
A method for depositing a very thin layer of phosphor powder for CRT screens and like applications is disclosed in U.S. Pat. No. 5,674,554. Again, although a laminate structure involving phosphorescent material is described, the aim is to produce a thin, solid layer of powder, in contrast to the present invention.